1. Field of the Invention
The present invention relates to porous glass and a process for its production.
2. Discussion of Background
Porous glass has an excellent separating ability since pores thereof are very much uniformly distributed. Therefore, it is expected as a prospective material for use as a separating filter material in the chemical industry or in the medical field, such as for gas separation, reverse osmosis, ultrafiltration or blood purification, as a carrier for enzymes and catalysts, or as a filler for chromatography. However, the skeleton of conventional porous glass is composed essentially of SiO.sub.2, and when used in an aqueous solution, it gradually dissolves, whereby the pore size increases. Further in an alkaline solution, it dissolves in a short period of time. Thus, it had a serious drawback in the durability.
It is known to add a substantial amount of an alkaline earth metal oxide to the matrix glass composition to produce porous glass having excellent alkaline resistance i.e. SiO.sub.2 --ZrO.sub.2 porous glass (Japanese Unexamined patent Publication No. 202839/1987, and Prepararatory papers for the annual lecture meeting of Yogyo Kyokai for 1986, p.493-494).
However, it is known that by this method, the amount of zirconia introduced into the glass skeleton is small relative to the amount of zirconia used. Further, the SiO.sub.2 --ZrO.sub.2 porous glass obtained by this method has a difficulty such that zirconia is present not in the glass skeleton but in pores of the glass skeleton in a substantial amount in the form of a zirconia gel, and it is difficult to obtain a porous glass unless treatment is applied to elute the zirconia gel by a strong acid.
Further, a method for producing SiO.sub.2 -ZrO.sub.2 porous glass by a sol-gel method is known (Japanese Unexamined patent Publication No. 17443/1986). However, the pore size of glass obtained by this method is as small as 40 .ANG. at the maximum.
It is also known to produce silicic acid having a large pore structure by impregnating an inorganic salt to silica gel, followed by calcining treatment (Japanese Unexamined patent Publication No. 5817/1972). However, this method teaches nothing about SiO.sub.2 -ZrO.sub.2 porous glass.
As glass fibers (non-porous) for reinforcing cement having excellent alkaline resistance, R.sub.2 O--ZrO.sub.2 --SiO.sub.2 glass wherein R is a metal element, Cem Fil ("Dictionary for Glass", compiled by Sumio Sakka, Asakura Shoten), is known.